1. Field of the Invention
The present invention relates generally to systems and methods for directing and controlling the operation of an induction motor.
2. Background of Invention
The “fuel” powering an induction motor, such as a field-oriented induction motor, is current. This current may be divided into two components, torque current and flux current. Torque current may be viewed as that component of the current which generates motive force, or torque. Flux current may be viewed as that component of the current which generates magnetic flux in the rotor of the motor. Shaft torque and rotor flux are related, with shaft torque proportional to the product of rotor flux and torque current. At high speeds, shaft torque may be calculated using back-EMF voltage.
Typically, in an induction motor, as the speed increases the back-EMF voltage increases as well. This reduces the voltage available to drive current into the motor, and thus limits the ability of the motor to produce torque. One well-known technique, field weakening, reduces the magnitude of the flux in the motor, thus reducing the back-EMF voltage and increasing the torque-producing capability of the motor.
Another well-known technique, overmodulation, involves the injection of harmonics into the fundamental current of the motor. These harmonics increase the overall size of the fundamental current. This allows for greater torque production than may be achieved with a pure sine wave drive, given a fixed DC bus voltage. One limitation of overmodulation, however, is that the fundamental current waveform becomes distorted and, as a consequence, nuisance trips of the overcurrent protection result.